TreeDragSource does not allow moving both the leafs and the nodes in same tree

Change: bug's title misses NOT word: "does NOT allow"

Current implementation does not allow moving both leafs and nodes in the same tree. Only option is possible in the same time (even in single selection mode). This is because TreeDragSource#onDragStart method tests TreeSource only for LEAF or NODE, and not BOTH. This makes hard to implement editable hierarchies.